This invention relates to methods of photo-detection for photo-detectors in photo-electronic devices such as static and video movie cameras and image sensors and for photo-telecommunication such as electronic notebooks and personal computers, as well as to optical detectors using such methods.
As an example of photo-detector incorporated, for example, in a remote-control cameras and video movie cameras, FIG. 4 shows a remote-control light-receiving circuit incorporated in a prior art electronic device. A photodiode PD.sub.10 is provided to serve as a light-receiving sensor in this light-receiving circuit, but the photo-current thereby obtained is due not only to a light signal from a light source (not shown) serving as a light emitter but also to external light. Since the photo-current due to the light source varies inversely proportional to the square of the distance between the light source and the light-receiving sensor, as shown in FIG. 6, a large photo-current can be obtained as the light-receiving sensor approaches the light source.
With prior art light-receiving circuits, therefore, it is often attempted to expand the dynamic range of the output current by compressing the photo-current as shown in FIG. 5 such that telecommunication within a range of distance between several centimeters and several meters can be carried out dependably. The light-receiving circuit shown in FIG. 4 comprises a current mirror circuit 10 with a first transistor Q.sub.11, to the collector of which is connected the photodiode PD.sub.10, and a second transistor Q.sub.12 opposite thereto, a resistor R being connected between the emitter of the second transistor Q.sub.12 and a power source V.sub.cc, thereby forming a current compression circuit of which the ratio of current compression is determined by the resistance of this resistor R. With a current compression circuit thus structured, it is possible, as shown in FIG. 5, to obtain an approximately logarithmically compressed output current A.sub.2, as the collector current of the second transistor Q.sub.12, from an input current A.sub.1 through the photodiode PD.sub.10.
With prior art light-receiving circuits, however, not only the photo-current due to external light but also the photo-current due to the signal light is compressed. When the external light is much stronger than the signal light, the signal current from the signal light becomes too weak. Table 1 shows an example which compares the compressed current, with and without fairly strong external light, when the signal current component flowing through the photodiode PD.sub.10 is 10 .mu.A. Let the current produced by the strong external light be 1000 .mu.A such that the total photodiode (PD) current A.sub.1 through the photodiode PD.sub.10 will be 1010 .mu.A and 10 .mu.A, respectively with and without the external light. FIG. 5 shows that the compressed current obtained therefrom will be 30.3 .mu.A and 10 .mu.A, respectively. Since the PD current through the photodiode PD.sub.10 due only to the external light will be 30 .mu.A according to FIG. 5, this means that the signal current after the compression is only 0.3 .mu.A (=30.3 .mu.A-30.0 .mu.A), which is too weak to be practically usable. In other words, prior art cameras and video movies could not be remote-controlled if the daylight was too strong.
TABLE 1 ______________________________________ With Without external external light light ______________________________________ Signal current (.mu.A) 10 10 External-light current (.mu.A) 1000 0 PD current (.mu.A) 1010 10 Total current 30.3 10 after compression (.mu.A) Signal current 0.3 10 after compression (.mu.A) ______________________________________
One of the methods of avoiding the effects of external light was to shorten the distance between the light source and the light-receiving sensor. A strong signal current can always be obtained by this method, independently of the intensity of the external light, but the use is severely limited to the immediate vicinity of the light source. This problem persists not only when electronic notebooks and personal computers of a laptop type are used outdoors for optical data telecommunication but also when they are used in the presence of an incandescent lamp.